The present invention relates to a photographic printer in which a CRT is used as a light source for making a print from an original.
There have been known photographic printers having CRTs as light sources for illuminating an original such as a negative film or a positive film from which prints are made. In such a photographic printer, the negative film is scanned with a flying light spot produced by the CRT. the luminance of which is controlled to transform the gradation, in particular the contrast, of an image to be printed. One such photographic printer is disclosed in. for example. Japanese Patent Publication No. 45-5336. The photographic printer disclosed in the above-mentioned Japanese Patent Publication is provided with a half mirror in the printing path between the negative film and a photographic material to reflect part of a flying spot produced by the CRT and passed through the negative film to provide a luminance control signal with which the flying light spot of the CRT is controlled in luminance.
Another photographic printer, disclosed in Japanese Unexamined Patent Publication No. 58-66,929 uses three separate CRTs for red, green and blue light, each of which scans a color original with a flying spot to determine the exposure, provide a particular illumination pattern, and print an image on a photographic material. In scanning for determining the exposure, the CRT is excited in such a way that the phosphor screen of the CRT emits light with a standard luminance. The light spot from the CRT is passed through the color original and detected separately by red, green and blue light measuring means so as to provide three color video signals. From the video signals, image characteristic values, for example a maximum density, a minimum density and a mean density are extracted to obtain the density difference between the maximum and minimum densities, by color. Then. a control signal is provided for each color to make the density difference coincide with a standard value. In scanning to provide the illumination pattern, the CRT is controlled with the control signal to provide a flying spot with a controlled luminance. The flying spot passed through the color original is measured by light measuring means, by color. to provide video signals which are memorized in a memory to provide a particular illumination pattern which is similar to a blurred image of the original. In scanning for printing, the color original is placed in contract with the CRT so as to be illuminated by the CRT. The luminance of the flying spot of the CRT is controlled according to the video signals read out from the memory and the exposure time is controlled according to the mean density. In this way, an image of the color original illuminated with the blurred image illumination pattern is exposed on the photographic material.
For making prints with well controlled color balance and gradation. a photographic printer may be provided with a color monitor to display the color image to be printed for simulation and visual inspection. Such a photographic printer with color monitor is disclosed in, for example, U.S. Pat. No. 4,668,082.
One problem with the prior photographic printer is that the use of high CRT luminance, although advantageous in order to reduce the exposure time results in long image persistency. Therefore, when causing the CRT to produce a high luminance flying spot for the purpose of reducing the exposure time, the video signal provided for each point of the original scanned by the flying spot is affected by the after image of the last scanned point, resulting in a low SN ratio of the video signal due to the persistency of the previous video signal.
Another problem in the prior photographic printer is the difficulty of gradation correction. This is due to the fact that, in the prior photographic printer, gradation correction is effected by transferring contrast. Furthermore, since gradation correction is made linearly with respect to the transmittance of an original, it is difficult to correct the gradation non-linearly in order to effect a correction (which is usually linear with respect to density) which is necessary to conform to human sensory perception. or to correct for under exposed or over exposed originals.
Further, the photographic printer with built-in monitor is used not only to make prints but to inspect the original before printing and is capable of correcting for changes in the luminance of a halogen lamp due to aging, because the halogen lamp is commonly used in printing and inspecting. However, it is difficult in this photographic printer to correct for the shading which is produced due to the gradation characteristic of the halogen lamp and/or the optical characteristic of the printing lens.